1. Field of the Invention
The present invention relates to novel processes and systems for the removal of impurities, such as sulfur compounds, hydrogen chloride, arsenic, selenium, hydrogen cyanide, ammonia, and combinations thereof, from a gas stream by a solid sorbent stream which is simultaneously regenerated to remove the impurities.
2. Description of Related Art
Removal of impurities from gas streams is desirable in numerous instances to address various process, environmental, chemical, and/or other industrial considerations as will be apparent to the skilled artisan. For example, “synthesis gas” or “syngas” produced by the gasification of fossil fuels such as coal, or other carbonaceous materials is becoming increasingly important as a viable alternative energy source and as an important raw material source for the industrial synthesis of various organic chemicals. However, syngas often contains various impurities, such as sulfur, arsenic, and selenium compounds, which are desirably removed in whole or part to facilitate subsequent processing and/or use of the gas. In particular, the gasification of coal, heavy oil fractions, and some types of carbonaceous waste, typically produces a syngas containing gaseous impurities such as hydrogen sulfide, carbonyl sulfide, hydrogen selenide, arsine, and the like. These impurities can be corrosive or toxic in some cases, and/or can act as catalyst poisons and/or environmental pollutants. There is a need, therefore, for methods to remove these compounds from synthesis gas streams in chemical processes to prevent damage to catalyst systems and to meet environmental standards.
Current commercially available processes for removing sulfur species from reducing gas streams, such as syngas, typically employ one of two methods: A) liquid phase absorption, either physical or chemical; or B) adsorption onto solid sorbents in fixed beds.
Syngas, such as that from the gasification of coal or other carbonaceous materials, generally exits a gasifier as a high temperature gas stream, typically at a temperature higher than about 900° F. (482° C.). Current liquid phase absorption methods for impurity removal are generally inefficient in the case of such high temperature gas streams because they typically operate at temperatures of about 100° F. (38° C.) or less. Therefore, large scale cooling and related heat recovery processing are necessary in the case of gasification syngas streams to allow removal of impurities at the lower temperatures required by liquid phase absorption processes. Such cooling, heat recovery and related processing steps result in thermal inefficiencies and substantial equipment cost as will be apparent.
Typically, solid sorbent, hot gas adsorption processes involve contacting a solid sorbent comprising an active metal oxide with hot gas to convert the active metal oxide into a metal compound comprising the impurity or a derivative thereof. The impurities may include, but are not limited to, sulfur, hydrogen chloride, arsenic, selenium, hydrogen cyanide, and/or ammonia. Desirable active metal oxide containing sorbent compositions and processes for sulfur removal are disclosed in U.S. Pat. No. 6,951,635 B2 issued Oct. 4, 2005 to Gangwal et al; U.S. Pat. No. 6,306,793 B1 issued Oct. 23, 2001 to Turk et al; U.S. Pat. No. 5,972,835 issued Oct. 26, 1999 to Gupta; U.S. Pat. No. 5,914,288 issued Jun. 22, 1999 to Turk et al; and U.S. Pat. No. 5,714,431 issued Feb. 3, 1988 to Gupta et al; which are each hereby incorporated herein by reference in their entireties.
Following the adsorption reaction and depending on the impurity, the impurity loaded sorbent is regenerated at high temperature. In other cases, the impurity loaded sorbent is discarded. If the sorbent is regenerated, hot gases containing impurities are typically produced during the regeneration step. In these cases the impurities can be typically separated from the regenerator off-gas for disposal or downstream processing. For example, in the case where syngas contains a sulfur impurity, regeneration of the sulfur loaded sorbent with an oxidizing gas stream, typically oxygen or an oxygen containing gas, produces sulfur dioxide which can be absorbed and/or converted to sulfuric acid, elemental sulfur or the like. In particular, the regeneration reaction converts the metallic sulfide back to metallic oxide via the following reaction:MS+3/2O2→MO+SO2  (I)wherein M is the active metal present in the sorbent, for instance Zn; MO represents a metal oxide; and MS represents a metal sulfide. The skilled artisan will understand that although oxidation is a preferred means of regenerating active metal oxide sorbents, other methods, such as thermal regeneration, also may be possible, particularly in the case of different solid sorbent compositions.
Fluidized bed adsorption and absorption/regeneration processes are known in the art and are disclosed for example in the previously identified US patent publications of Gangwal et al, Gupta et al, and Turk et al. Coupled fluidized bed reactor/regeneration systems are also known and used in the processing of hydrocarbons, for example in Fluid Catalytic Cracking (FCC) processes.
Dual loop fluidized bed absorption/regeneration processes for removing sulfur contaminants from hydrocarbon gases such as syngas, are disclosed in U.S. Pat. Nos. 5,447,702 and 5,578,093, issued Sep. 5, 1995 and Nov. 26, 1996, respectively, to Campbell et al, which are hereby incorporated herein by reference. In such dual loop processes, absorption and sorbent regeneration are simultaneously conducted in coupled fluidized beds. In these dual loop processes, the solids flow rate of the sorbent through the absorber can be different than the solids flow rate of the sorbent through the regenerator. In particular, the sorbent stream exiting the absorber can be separated into two streams, a recycle stream which is recycled to the absorber, and a regeneration stream which is passed to the regeneration zone for removal of absorbed sulfur. The regenerated sorbent stream exiting the regenerator is returned to the absorber where it is mixed with the recycled sorbent. However, in order to achieve steady state operation and establish equilibrium between absorption and regeneration in such dual loop processes, the quantity of sulfur removed from the sorbent in the regenerator must match the quantity of sulfur removed from the feed gas in the absorber. In turn, since the quantity, i.e., flow rate, of sorbent solids passing through the absorber exceeds the quantity of sorbent solids passing through the regenerator, the sulfur pick-up in the absorber, as a percentage of sorbent weight, must be lower than the sulfur removal rate in the regenerator, based on sorbent weight.
In practice, long term, steady state operation of the dual loop absorber/regenerator fluidized bed reactor systems disclosed in Campbell et al. can present problems since process changes in either loop must be accompanied by corresponding changes in the other loop in order to maintain stable continuous operation. For example, variations in the composition, feed rate, temperature, pressure, etc., of the feed gas fed to the absorber can cause long term and short term variations in the rate of sulfur removal from the feed gas in the absorber (with a corresponding change in weight percent sulfur pick-up by the sorbent, based on sorbent weight), requiring corresponding process and/or sorbent flow rate variations in the regenerator for the maintenance of stable continuous operation. Moreover, conventional mechanical valves such as the solids plug valves disclosed by Campbell et al. as a means for varying the flow rates of sorbent solids to the absorber and/or regenerator, are subject to erosion, plugging and other problems due to the high temperature, high pressure, and corrosive and abrasive conditions inherent in the desulfurization processes disclosed by Campbell et al.